Strain gauge on a flexible support and transducer equipped with said gauge

ABSTRACT

Strain gage disposed on a flexible support and probe fitted with the gage. A gage with its creep being adapted according to the test body and the application required, without having to change the mask for etching the strain-sensitive thin film. A strain gage fitted to one of the surfaces of a test body capable of deformation under the action of a quantity to be measured. The gage comprises a strain-sensitive thin film (22) etched in the form of a resistance and fitted to a flexible support (20), the film (22) comprising at least two underlying films (24, 26) having different creep values.

DESCRIPTION

The present invention relates to a strain gauge on a flexible support,as well as to a transducer equipped with said gauge.

Strain gauges on a flexible support are used for measuring thedeformations of mechanical parts. They have numerous applications. Theyare in particular used for weighing (e.g. commercial and bathroomscales), for pressure measurements, for stress measurements onmechanical parts such as transmission shafts, different portions of anaircraft wing or concavities. They are also used in extensometry for theinspection of concrete structures, such as dams or bridges. Finally,these gauges can also be used for performing torsion, torque, vibrationor acceleration measurements.

In its most simple form, the gauge 1 is constituted by a very thinstrand 3 bonded to a thin support 5 and arranged in the form of loopsshown in FIG. 1, i.e. most of its length is located parallel to a fixeddirection (arrow X). Larger strands 7 are used for welding the outletsfor cables for connecting to instruments.

When it is wished to know the elongation of a structure in a givendirection, bonding takes place of the gauge 1 with the strands parallelto said direction. The gauges 1 are also used for producing transducers9, such as is illustrated in the attached FIG. 2. A transducer is amechanical device for transforming a random physical quantity A(pressure, force, acceleration, etc.) into a deformation of a part 11known as the test body. Gauges bonded to test bodies detect itsdeformations with a view to the measurement of the physical quantity Aor for acting on regulating devices.

In the case illustrated in FIG. 2, the two gauges 1c operate incompression and the two gauges 1e in extension. This makes it possibleto obtain very accurate measurements of approximately 10⁻⁴ of the extentof the measurement. The same arrangement can be used on all types ofstructures, as described in "L'encyclopedie Vishay d'analyse descontraintes", Vishay-Micromesures, Malakoff, France, 282-284.

Finally, the gauges can be arranged in the form of a Wheatstone bridge,as is described e.g. in EP-A-53 059.

No matter what the arrangement of the strain gauges, the measurementsperformed are all based on resistance variations of the strand 3, saidvariations being a function of the type of material forming it or itslength and section formations.

EP-A-53 059 discloses a strain gauge, obtained by the vacuum depositionof a metal alloy coating of 50 to 500 nm on a 100 to 250 μm thick glasssubstrate. However, glass is extremely brittle and is difficult to usein certain cases. Therefore this gauge can be used only in compressionand not in tension due to the ultimate strength of the glass.

The prior art also discloses strain gauges to be used in compression andextension and which for this purpose are fixed to a flexible support.These gauges have a thin film of polyimide or phenolic epoxy resin witha thickness of 25 μm and to this is bonded a very thin sheet ofapproximately 5 μm of a laminated resistive material such as an alloy ofnickel-chromium, nickel-copper or platinum-tungsten.

In this case, the resistive layer is then etched in fine strip form inorder to obtain a resistor, whose shape can be gathered from theenclosed FIG. 1. The resulting resistors can have values between 120 and6000 ohms.

This type of gauge suffers from a certain number of disadvantages. It isdifficult to manufacture the gauges, because the bonding of very thinsheets of resistive material is a complicated operation, which it isdifficult to reproduce. The manufacture of 5 μm thick metal sheets alsotakes a long time and is difficult, because it requires a succession ofrolling and annealing operations for stabilizing the material betweeneach rolling stage. Finally, the thickness of the 5 μm resistive metalcoating limits the values of the resistors obtained, which generally donot exceed 6000 ohms.

U.S. Pat. No. 4,786,887 discloses a gauge having a flexible substratecovered with a polymeric insulating layer and a resistive, nickel -chromium alloy layer, a gold conducting layer also being placed solelyon the outlet tabs in order to form contact studs. This documentproposes adjusting the creep of the test body by modifying thecharacteristics of the insulating layer.

Moreover, under the action of a constant force, the test body 11 and thegauge 1 instantaneously deform at the time of applying the said force,but continue to progressively deform over a period of time i.e. theso-called creep phenomenon. When the force is removed from the test body11, the latter returns to its initial position. The creep value ismeasured by forming the ratio between the length variation of theelement subject to creep and its initial length.

In the same way, following the instantaneous deformation of the gauge 1,which follows that of the test body 11, the gauge 1 is subject to aforce opposing said deformation and this constitutes the so-calledrelaxation phenomenon. The latter corresponds to a reduction of thestress or strain exerted on the gauge, when the deformation is keptconstant.

In general terms, a gauge 1 fixed to a test body 11 to which is applieda load A is subject to three different deformations:

an instantaneous deformation corresponding to the load application,

a deformation due to the creep of the test body and

a deformation due to its inherent relaxation.

The result of the measurement performed with the aid of the gaugecorresponds to the resultant of these three deformations. However, thecreep or relaxation characteristics are adapted as a function of theapplications of the gauges.

In the case of the strain gauge shown in FIGS. 1 and 2, the transmissionof the deformations of the test body 11 to the gauge I takes place bycutting the connecting loops 15 between the successive strands 3, at theends thereof.

When it is wished to measure the evolution of the creep of a structureunder a constant load, such as e.g. a bridge, the gauge must be withoutrelaxation. However, the relaxation of the gauge 1 is dependent on thelength of the loops 15, the shorter the loops 15 the greater therelaxation of the gauge. Thus, the relaxation of the gauge is adapted bychoosing the length of the loops. This makes it necessary, prior tomanufacture, to calculate the length of the loops for each test bodyused. It is then necessary to have one gauge designed per test body andone etching mask per test body. Therefore the manufacturing process isonerous.

EP-A-53 059 makes no reference to the problem of regulating the creep,because the sought application is mainly general public weighing(bathroom and domestic scales), where the necessary accuracy is lessthan in the case of the weighing measurements performed in theprofessional sector. For general public transducers, the errors due tocreep fall within the measurement tolerance ranges.

However, when the gauges are used for a precise measurement, the designof the gauge 1 must make it possible to have a relaxation perfectlycompensating the creep of the test body 11, so as to have a constantoutput signal. Such an accuracy is necessary to ensure that the weightreading is constant, no matter what the duration of the weighingoperation.

U.S. Pat. No. 4,876,893 also discloses a strain gauge for a pressuretransducer. This gauge comprises an electrically insulating, base plate(insulated metal or glass), covered with a single thin alloy film. Thepreferred composition of said alloy is as follows: (Ni_(a)Cr_(100-a))_(100-b) Sib with a between 40 and 60% and b between 3 and 8%by weight. However, this document makes no reference to the creepproblem.

In general terms, the creep and relaxation phenomena are not veryimportant at ambient temperature, but may no longer be negligible whenthe test body and/or adhesive bonding the gauge to said body are heatedto temperatures close to their use limits. This effect is reduced bymaking the assembly undergo a heat treatment at a temperature higherthan that of the subsequent use. This is important, particularly in thecase of transducers which must have a fidelity better than 0.1%.However, such heat treatments are expensive.

Moreover, it is possible for the test body 11 to have an expansioncoefficient differing very significantly from that of the gauge 1. Asthe bonding was definitive at the adhesive treatment temperature, onreturning to the ambient temperature the gauge is subject to adeformation. Therefore the true zero does not correspond to the loadabsence case, but instead to that of a load under the bondingconditions. Thus, certain installations creep in the absence of a load,but not for a given deformation.

Finally, it is known that a resistance can vary as a function of thetemperature in accordance with the following formula:

    R=Ro(1+αT)

in which Ro represents the value of the resistance of the gauge at areference temperature, T represents the temperature at the time of themeasurement and α represents the temperature coefficient of resistance(TCR) of the material in which the resistor is produced (strands 3).When the TCR is close to zero, the resistance value does not vary as afunction of the temperature.

It would therefore be desirable to produce gauges from materials havinga TCR close to zero.

The metrological qualities of a gauge and a transducer are largelydependent on the control of the creep phenomena.

Therefore the invention aims at obviating the aforementioneddisadvantages and more particularly at permitting the adaptation of thecreep of the gauge to different test bodies, whilst maintaining atemperature coefficient of resistance close to zero and without it beingnecessary to modify the design of the mask making it possible to producethe loops for each test body.

The invention therefore relates to a strain gauge for fixing to one ofthe faces of a test body able to deform under the action of a quantityto be measured, said gauge incorporating a thin film sensitive todeformations etched in resistor form and fixed to a flexible support.

According to the features of the invention, said thin film incorporatesat least two underlying films having different creep values.

Advantageously, the film has a multifilm structure with several filmshaving different creep values and different thicknesses.

Thus, it is possible to adapt the degree of relaxation or creep of thegauges by varying the nature of the thin films, their number and theirthickness.

Advantageously, one of the underlying films has a positive creep valueand it is an alloy in the amorphous state with a TCR close to zero,whereas the other underlying film has a negative creep value and it isan alloy in the crystalline state, whose TCR is also close to zero.

The invention also relates to a transducer for measuring a quantity.According to features of the invention, it comprises at least one straingauge according to the invention fixed to a test body able to deformunder the action of the said quantity to be measured.

The invention is described in greater detail hereinafter relative tonon-limitative embodiments and with reference to the attached drawings,wherein show:

FIG. 1 A diagram illustrating a prior art strain gauge in plan view.

FIG. 2 A perspective view of a transducer having several prior artgauges.

FIG. 3 A part perspective view of an embodiment of a strain gaugeaccording to the invention.

FIG. 4 A curve illustrating the creep as a function of time for straingauges according to the invention and control gauges.

According to a first embodiment of the invention, the strain gaugeillustrated in FIG. 3 has an elongated shape identical to that describedin detail relative to FIG. 1. This strain gauge has a flexible support20 preferably made from a thermosetting polymer and able to resisttemperatures of at least approximately 400° C., such as a polyimide.

This flexible support 20 is covered with a deformation-sensitive thinfilm 22, which is in the form of the resistor described relative toFIG. 1. According to the characteristics of the invention, itincorporates at least two underlying films 24, 26 having different creepvalues.

The first underlying film 26 is preferably made from an alloy having apositive creep value and in the amorphous state. This underlying film ismade from a material chosen from among alloys based on nickel-chromium,platinum-tungsten or copper-nickel. Advantageously the alloy comprisesnickel, chromium and silicon. More specifically, it has the followingformula: Ni_(x) Cr_(y) Si_(z) with 5<z<11 and x+y+z=100. Even morespecifically, it comprises by weight, approximately 72% nickel, 18%chromium and 10% silicon. Its temperature coefficient of resistance TCRis close to zero.

The second underlying film 24 is preferably constituted by an alloyhaving a negative creep value, i.e. which leads to a significantrelaxation of the gauge compared with the test body on which it isplaced. Preferably, said alloy is in the crystalline state. It is chosenfrom among alloys based on nickel-chromium, platinum-tungsten orcopper-nickel. Advantageously, it is constituted by constantan, i.e. analloy containing by weight, approximately 55% copper, 44% nickel and 1%manganese. Its temperature coefficient of resistance (TCR) is close tozero.

It is possible to use two constantan types, doped by several impurities,the most important of which are given in the following Table 1. It wouldalso be possible to use zinc, silver or titanium as the dopant.

                  TABLE 1                                                         ______________________________________                                                 constantan no. 1                                                                        constantan no. 2                                                    (μg/g) (μg/g)                                                  ______________________________________                                        Ca         935         500                                                    Pb         200         175                                                    Si         200          25                                                    Fe         420         215                                                    Al          60          25                                                    Mg          55         615                                                    ______________________________________                                    

The first constantan alloy (no. 1) has a TCR close to 0. The secondconstantan alloy (no. 2) has a higher TCR, but a lower relaxation.

A constantan alloy having a low impurity level has a higher TCR and viceversa. Thus, the quantity of dopants present in the alloy will beadapted as a function of the sought TCR.

Advantageously and as shown in FIG. 3, preferably the alloy underlyingfilm 26 in the amorphous state is directly deposited on the flexiblesupport 20 and then the underlying film 24 is deposited on theunderlying film 26. This can also be reversed, but the results obtainedare then less homogeneous.

It should be noted that it is not vital for one of the underlying filmsto have a positive creep value and for the other to have a negativecreep value and it is merely sufficient for these values to bedifferent.

The degree of creep of the gauge or more precisely the film 22 isdependent not only on the value of the creep relative to each of theunderlying films 24, 26, but also the relative thickness and the numberof underlying films 24, 26. The tests described hereinafter wereperformed with three and five films.

The manufacturing process for the gauges will now be described ingreater detail. Multifilm deposits of amorphous (NiCrSi) and crystalline(CuNiMn) alloys took place by cathodic sputtering with a continuousdiode using a 25 μm thick, flexible polyimide support 20. This type ofdeposition process is used because the polyimide polymer must be able towithstand temperatures of 400° C. or higher, reached during theproduction of the films. Obviously, this temperature value can belowered by adding cooling systems to the deposition apparatus. Thechoice of the polymer is dependent on the temperature reached during thedeposition of the thin films. This choice falls within the scope of theexpert. These thin film deposition methods make it possible to depositfilms having thicknesses between 50·10⁻¹⁰ m and 10000·10⁻¹⁰ m and tohave very high resistances per length unit. These resistances orresistors are etched by chemical etching following the geometry of FIGS.1 or 3 using a single mask for the different multifilm arrangements(i.e. a single loop length 15). For example, the connectors 28 are thendeposited through a mask at the two ends of each gauge by vacuumevaporation of a thickness of 100 Å chromium, 3000 Å nickel and 3000 Ågold. Finally, the gauges obtained are bonded to the test body so as toform a transducer. In the manner illustrated in FIG. 2 they can bearranged in Wheatstone bridge form. Connecting wires are then welded tothe connectors using a soldering iron and e.g. a tin - lead alloy.

Test performed on strain gauges having the structure according to theinvention.

The gauges were installed on a test body in a Wheatstone bridgearrangement. The test body is intended to perform a weighing operationbetween 0 and 3 kg. This test body has a creep with an intermediatevalue with respect to all the test bodies generally used for weighingpurposes. FIG. 4 illustrates the measurements performed applying amaximum load and taking unbalanced readings of the Wheatstone bridge for30 minutes. The curves represent the creep (i.e. the deformation in.permill. of the maximum deformation), as a function of time. The testswere performed with a gauge having a flexible polyimide support coveredwith one or more crystalline constantan or amorphous NiCrSi underlyingfilms. In front of each curve is also shown the corresponding section ofthe gauge without the support film 20 and the relative thicknesses ofthe constantan and the NiCrSi alloy.

Curve C1 represents the resultant obtained with a single constantan filmand forms a control. The creep is -1·10⁻³ after 30 minutes.

Curves C2, C3 and C4 demonstrate the reduction of the relaxation effect(increase of the creep value) due to the increase in the thickness ofthe underlying NiCrSi film compared with that of constantan. Curve C4corresponds to a creep of the gauge which precisely compensates thecreep of the test body.

Curves C5, C6 and C7 represent the results displaced towards thepositive creep values, obtained with alternating underlying films ofNiCrSi alloy and constantan. It should be noted that the same curvescould be obtained with only two underlying films by increasing thethickness of the NiCrSi alloy compared with that of the constantan.

Finally, for comparison, curve C8 shows the creep of the test bodyobtained with a NaCrSi alloy film only. This value is 1.4·10⁻³ after 30minutes. In this case, the metal film perfectly follows the test bodyand the gauge has no relaxation. The strain gauges according to theinvention have a particular application in precision weighing forweight/price scales.

As a function of the particular application and especially the desiredcreep value, a choice will be made of the number and the thickness ofthe different alloy underlying films.

We claim:
 1. Strain gauge for fixing to one of the faces of a test body able to deform under the action of a quantity to be measured, said gauge having a thin film (22) sensitive to deformations, etched in resistor form and fixed to a flexible support (20), characterized in that said thin film (22) has at least two underlying films (24, 26) having different creep values.
 2. Strain gauge according to claim 1, characterized in that the flexible support (20) is made from a polyimide able to resist temperatures equal to or higher than approximately 400° C.
 3. Strain gauge according to claim 1, characterized in that alloy underlying film (26) having a positive creep value is in direct contact with the flexible support (20).
 4. Strain gauge according to claim 1, characterized in that each underlying film (24, 26) having different creep values has a thickness between 50·10⁻¹⁰ m and 10000·10⁻¹⁰ m.
 5. Strain gauge according to claim 1, characterized in that the thin film (22) has a multifilm structure with several underlying films (24, 26) having different creep values and different thicknesses.
 6. Transducer for measuring a quantity, characterized in that it incorporates at least one strain gauge according to claim 1 fixed to a test body (11) able to deform under the action of said quantity to be measured.
 7. Strain gauge according to claim 1, characterized in that one of the underlying films is constituted by an alloy (26) having a positive creep value and the other underlying film of an alloy (24) having a negative creep value.
 8. Strain gauge according to claim 7, characterized in that the alloy having a positive creep value (26) is an alloy in the amorphous state.
 9. Strain gauge according to claim 8, characterized in that the alloy in the crystalline state and the alloy in the amorphous state are selected from the group consisting of alloys based on nickel-chromium, platinum-tungsten and copper-nickel.
 10. Strain gauge according to claim 8, characterized in that the alloy has a temperature coefficient of resistance close to
 0. 11. Strain gauge according to claim 4, characterized in that the alloy having a positive creep value (26) is an alloy of composition Ni_(x) Cr_(y) Si_(z) with 5<z<11, x+y+z=100.
 12. Strain gauge according to claim 11, characterized in that the alloy (26) has by weight approximately 72% nickel, 18% chromium and 10% Si.
 13. Strain gauge according to claim 7, characterized in that the alloy having a negative creep value (24) is an alloy in the crystalline state.
 14. Strain gauge according to claim 13, characterized in that the alloy has a temperature coefficient of resistance close to
 0. 15. Strain gauge according to claim 13, characterized in that the alloy (24) having a negative creep value is doped with an element selected from the group consisting of calcium, lead, silicon, iron, aluminum, magnesium, zinc, silver and titanium.
 16. Strain gauge according to claim 12, characterized in that the alloy (24) having a negative creep value is a copper, nickel and manganese alloy.
 17. Strain gauge according to claim 16, characterized in that the alloy (24) contains by weight approximately 55% copper, 44% nickel and 1% manganese.
 18. Strain gauge according to claim 13, characterized in that the alloy in the crystalline state (24) and the alloy in the amorphous state (26) are selected from the group consisting of alloys based on nickel-chromium, platinum-tungsten and copper-nickel.
 19. Strain gauge according to claim 18, characterized in that the alloy (24) having a negative creep value is doped with an element selected from the group consisting of calcium, lead, silicon, iron, aluminum, magnesium, zinc, silver and titanium.
 20. Strain gauge according to claim 18, characterized in that the alloy having a positive creep value is an alloy of composition Ni_(x) Cr_(y) Si_(z) with 5<z<11,x+y+z=100.
 21. Strain gauge according to claim 20, characterized in that the alloy (26) has by weight approximately 72% nickel, 18% chromium and 10% Si. 